The present invention relates to an organic electroluminescent display device and a method of forming the same, and more particularly to an organic electroluminescent display device suitable for a flat display and a method of forming the same.
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional organic electroluminescent display device. The organic electroluminescent display device is formed on a transparent supporting substrate 31. Usually, a transparent electrode made of indium tin oxide is first patterned on the transparent supporting substrate 31 by use of a photo-lithography and subsequent wet etching in chemicals including ferric chloride. The patterned indium tin oxide transparent electrode 32 serves as an anode. An organic electroluminescent layer 35 is deposited on the patterned indium tin oxide transparent electrode 32 by a vacuum evaporation method. A cathode 37 is then patterned on the organic electroluminescent layer 35. If the cathode 37 is patterned by a wet etching which uses a photo-resist technique, it is required to remove the used photo-resist film from the cathode 37 after the wet etching. This causes the following problem.
When the used photo-resist film is removed from the cathode 37 and also when the etching to the cathode is made by the photo-resist, a moisture infiltration appears onto an interface between the organic electroluminescent layer and the cathode. This moisture infiltration causes remarkable deterioration in luminescent property and life-time characteristic.
In order to avoid those problems, a shadow mask may be formed for evaporations of a plurality of organic electroluminescent materials and the cathode to form the organic electroluminescent display device.
If the shadow mask is used as illustrated in FIG. 1, a transparent electrode made of, for example, indium tin oxide is deposited by a sputtering method on the transparent supporting substrate 31 of glass for subsequent patterning of the transparent electrode into a strip shape anode 32. Then, first and second organic electroluminescent layers 33 and 34 are laminated entirely over the anode 32 and the transparent supporting substrate 31. The laminations of the first and second organic electroluminescent layers 33 and 34 form an organic electroluminescent layer 35.
A shadow mask made of a metal is prepared which has a striped shielding portion 36 with stripe-shaped slits. This shadow mask is placed so that the stripe-shaped slits have a longitudinal direction perpendicular to the direction of the anode pattern and the shadow mask is made into contact with the organic electroluminescent layer 35. Thereafter, the cathode material is deposited by a vacuum evaporation onto the organic electroluminescent layer 35 through he shadow mask.
In Japanese laid-open patent publication No. 5-275172, there is disclosed an organic electroluminescent image display and a method of manufacturing the same. A plurality of indium tin oxide anode lines are aligned over the transparent supporting substrate at an interval in a lateral direction. Walls are provided which are distanced in a direction perpendicular to the anode lines before the organic electroluminescent layer is formed on the transparent supporting substrate surface having the indium tin oxide anode lines and the walls. The walls have a height which is higher than the thickness of the organic electroluminescent layer. Then, a metal vapor phase deposition source is set at such an angle that an isolation wall is inserted between the source and an adjacent portion to the organic electroluminescent layer surface whereby a metal film serving as a cathode is formed.
As a material of the walls, a spin-coated negative photo-resist film or a dry film is available. The photo-resist or the dry film is exposed to an optical pattern so that the exposed region of the photo-resist shows a cross-linking reaction and to be insoluble whilst the unexposed region of the photo-resist is developed and cleaned to be removed whereby the patterned walls of photo-resist are made.
Alternatively, there is disclosed the following method of forming the walls. A photo-resist is provided by patterning process on a region surrounding an area on which walls are intended to be formed later. Subsequently, a wall material such as silica, silicon nitrite or alumina is applied on the above region. Thereafter, the photo-resist is removed by a lift-off method to form the walls.
If a metal for a cathode is deposited by a vacuum evaporation at an oblique angle, then the metal is isolated along the walls at a potion where the wall is sandwiched between that portion and the organic electroluminescent layer. As a result, the cathode can be formed which has the desired pattern.
A voltage in the range of 5-20 V across the anode and the cathode is applied to between the anode and cathode to cause a current through the organic electroluminescent layer whereby the organic electroluminescent display device formed in the above manners is driven.
The above conventional organic electroluminescent display device has such a structure that the organic electroluminescent layer is sandwiched between the anode and cathode. A voltage is applied between the anode and cathode to cause luminescence of the organic electroluminescent layer for which reason in order to display a desired pattern, patterning of the anode and cathode is required. In a dot-matrix display, the patterning to the anode and cathode is so made that the anode and cathode cross each other in a matrix.
As described above, the patterning to the anode on the transparent supporting substrate is relatively easy. The problem is in the patterning to the cathode on the organic electroluminescent layer. If the wet etching method using the photo-resist technique is used, then the cathode is selectively etched and thereafter the photo-resist is removed. At those times, a moisture infiltration appears onto an interface between the organic electroluminescent layer and the cathode, whereby the luminescent property and the life time of the display are remarkable deteriorated.
To avoid the above problem, it had been proposed to use a shadow mask for patterning the cathode on the organic electroluminescent layer. It had also been proposed to use the patterning to the cathode by use of the photo-resist walls as masks.
In the method using the shadow mask, if, however, the substrate has a large size, then the shadow mask is not securely fitted on the organic electroluminescent layer at a center portion of the transparent supporting substrate due to a weight of the shadow mask whereby it is difficult to obtain a desired and accurate pattern of the cathode. This might form a short circuit of the cathode. Even if the substrate size is not so large but a fine pattern is required, then it is required that the shadow mask be thin. Such a thin shadow mask has a small rigidity whereby it is difficult to securely contact the shadow mask with the organic electroluminescent layer.
To attempt to settle the above problem, it had been proposed that a thin shadow mask of a magnetic material be formed and a magnet be placed on an opposite surface to the surface of the transparent supporting substrate on which the organic electroluminescent layer has been provided, so that stripe shaped portions of the shadow mask are forced into secure contact with the organic electroluminescent layer by a magnetic force. It might, however, be possible that the shadow mask makes a scratch 38 on the organic electroluminescent layer before the shadow mask is made to securely contact the organic electroluminescent layer. The thickness of the organic electroluminescent layer is thinner than 1 micrometer to suppress the driving voltage at not more than 20 V, for which reason the scratch 38 might reach the anode beneath the organic electroluminescent layer. If the scratch 38 exits at a position where a cathode pattern is intended to be formed, a metal for cathode is made into contact with the anode 32 through the scratch 38 whereby a short circuit is then formed between the anode and cathode. The short circuit between the anode and cathode prevents the necessary voltage from being applied to the organic electroluminescent layer sandwiched between the cathode and anode. As a result, no influence appears on the display pixel located at a position where the short circuit is formed. This causes a drop of the yield of the display.
In the later method using the photo-resist walls, it is required to provide walls between the organic electroluminescent layer and the an evaporation source for deposition of a metal for the cathode, for which reason a large vacuum evaporator is required for patterning a cathode over a large size transparent supporting substrate. This requires a large cost for the equipment. The manufacturing cost is therefore increased.
Adjacent two cathodes are distanced at a distance which corresponds to the sum of a width of the wall and a width of a region projected onto the organic electroluminescent layer surface along the walls sandwiched between the evaporation source for evaporating a metal of cathode and the organic electroluminescent layer. For this reason, in case of the large size display substrate, the separation width of the pixels onto which the walls are projected is different between positions near and far the evaporation source for evaporating the metal of cathode.
Further, if the patterns extend almost in parallel to a straight line connecting between the evaporation source for evaporating the metal of cathode and the transparent supporting substrate, then the wall has no part which is projected onto the organic electroluminescent layer. This means it impossible to pattern the cathode over the organic electroluminescent layer.
In the above circumstances, it had been required to develop a novel organic electroluminescent display device free from the above problems and disadvantages.